Pressure-fired fluidized-bed boiler installation

ABSTRACT

The pressure-fired fluidized-bed boiler installation with a gas turbine set driven by the flue gases has, in the main air duct (8), a preheating burner (10) which heats the still cold fluidized-bed boiler (1) and a part of the flue gas duct (12) to a temperature above the dew point. Air branched off from the main air duct (8) is heated in a heat exchanger (20) and used for heating up the turbine (5) driven by the motor (6) and the remaining parts of the installation, through which flue gas flows.

The present invention relates to a pressure-fired fluidised-bed boilerinstallation according to the preamble of patent claim 1.

In an endeavour to obtain better thermal utilisation of coal as the fuelfor industrial furnace installations, numerous concepts of fluidised-bedfurnaces have been proposed in recent years. As far as is known,however, proposals in this direction, especially for pressure-firedfluidised-bed boiler installations for power stations, have not yet gonebeyond the project stage.

The individual problems, which must also be solved before fluidised-bedfurnaces are feasible in practice, include the provision of a devicewhich prevents the incrustation which is caused by the mixing ofcondensate from still cold flue gases with dust deposits which havesettled during operation of the installation on the components alongwhich the flue gas flows. The flue gases from a pressure-firedfluidised-bed boiler installation contain a considerable quantity ofdust which must be precipitated in a dust precipitation device to suchan extent that corrosion and erosion of the components are restricted toa technically and economically acceptable level. A part of the fine dustwhich still remains in the flue gases after this precipitation forms thesaid deposits, especially in the pipes and filters and on the blading ofthe gas turbine for driving the blower which generates the compressedcombustion air for the boiler.

When the cold boiler installation is started up after a prolongedinterruption in operation, the formation of condensate in the start-upphase must therefore be avoided. Otherwise, the tacky film, which isformed by bonding of the condensate, separating out of the stillinsufficiently warm flue gases, to the fine dust deposits and which isvery difficult to remove, would in the course of time impair thefunctioning of the installation, in particular that of the shut-offelements and the turbine blading. To prevent this, the components of theinstallation according to the invention, as characterised in the patentclaims, are preheated before starting up to such an extent that thewater vapour present in the flue gases cannot cool down to a temperaturebelow the dew point and condense.

The single FIGURE of the drawing diagrammatically shows a fluidised-bedboiler installation with an illustrative example of a preheating deviceaccording to the invention.

In a fluidised-bed boiler 1 pressure-fired with small coal, steam isgenerated on a boiler heating surface 2, for example for a stem powerstation. The small coal is jetted into the pressurised firing space bymeans of a feeding device 3. The pressure is generated by an aircompressor 4 which is coupled to a gas turbine 5 driven by the fluegases. In addition, the turbine 5 is also coupled to a generator 6 whichcan also be run as a motor.

In normal operation, with the installation warm, the compressor 4delivers air under pressure into the boiler in which the small coal isburned in a fluidised bed 7. In normal operation, that is to say at asteady operating temperature, the compressed air passes through a mainair duct 8 via a control valve 9 into a preheating burner 10 which canbe heated by oil or gas. If the bed temperature is insufficient forstarting up, heating to the requisite temperature is effected with oilor gas fed via a fuel line 11.

The flue gases escaping from the boiler 1 in normal operation passthrough a flue gas duct 12 into a dust precipitation device with a dustfilter 13 and a dust discharge lock 14, from where the dust precipitatedin the filter is discharged into the open. The flue gases purified to ahigh degree in the dust precipitation device flow through a further fluegas duct 15 into the turbine 5, where they produce work, whereupon theyare discharged through an off-gas duct 16 into the open.

Upstream of the control valve 9, a heating air duct 17 branches off fromthe abovementioned main air duct 8 and leaves via a control valve 19dependent on a governor 18 into a heat exchanger 20, which can be heatedpreferably by steam or electrically, and further into the flue gas duct15 in the vicinity of the exit of the latter into the turbine 5.

A heating air discharge line 21 with an isolation valve 22 branches offfrom the flue gas duct 12 and leads into the off-gas duct 16. As far asis possible and necessary, steam-heated or electric heating coils 23 areprovided on the components of the installation through which heating airflows, such as the flue gas ducts 12 and 15, the dust filter 13 and thelock 14 as well as the off-gas duct 16, in order to keep the heating-upperiod of the installation as short as possible.

During heating-up, the turbine 5 and the compressor 4 are driven by thegenerator 6, connected as a motor, at a speed of rotation which isgreatly reduced as compared with the nominal speed of rotation in normaloperation, preferably at about 30% of the nominal speed of rotation. Theair delivered by the compressor is divided into two part streams byappropriate setting of the control valves 9 and 19. The main streamflows through the main air duct 8 via the control valve 9 into thepreheating burner 10, the combustion gases of which heat the coldfluidised bed, the boiler 1 and that part of the flue gas duct 12 whichadjoins the boiler 1 up to the isolation valve 22. The latter is open,so that the cooled combustion gases flow out via the heating airdischarge line 21 into the off-gas duct 16. The smaller part streamflows through the heating air line 17 via the open control valve 19 intothe heat exchanger 20, the throughput of which can be matched to thedesired heat release to the heating air by means of a control valve 24,and onwards into the flue gas duct 15 between the dust precipitationdevice and the turbine 5. A part of this heating air flowing into theflue gas duct 15 flows through and heats the turbine driven by the motor6, whereupon it leaves the installation through the off-gas duct 16. Theother part of the heating air takes a path through the flue gas duct 15back into the dust filter 13 and the flue gas duct 12 between the boiler1 and the filter 13 and passes via the valve 22 and the heating airdischarge line 21 likewise into the off-gas duct 16 and from there intothe open. The controller 19 regulates the air flow through the line 17as a function of the speed of rotation of the turbine rotor. In order toobtain refined control, the servo motor of the control valve can also besubject to suitable temperature signals from other points in the heatingair circulation, in order to ensure that the temperature is everywhereabove the dew point.

After all the parts of the installation which have to be preheated havereached a temperature above the dew point, the control valves 19 and 22in the heating air duct 17 and the heating air discharge line 21respectively are closed and the heat exchanger 20 is switched off. Theinstallation can then start power operation with small coal firing.

I claim:
 1. Pressure-fired fluidised-bed boiler installation, with afluidised-bed boiler fired with small coal for generating steam forenergy, heating or process purposes, with a gas turbine set which iscoupled to a generator, which can be connected as an electric motor, andthe compressor of which is connected via a main air duct to thefluidised-bed boiler and the gas turbine of which is connected via aflue gas duct with an interposed dust precipitation device to thefluidised-bed boiler, the dust precipitation device having a dust filterand a dust discharge lock, and also with an off-gas duct for dischargingthe flue gases, expanded in the gas turbine, into the open,characterised by a preheating device which is intended to preheat to atemperature above the dew point of water those parts of the installationwhich are at risk, when the boiler installation is started up after aninterruption in operation, due to mixing of the water vapour condensingout of the flue gases with deposits of fine dust, and which essentiallycomprises the following components:a preheating burner (10) located inthe main air duct (8) upstream of the fluidised-bed boiler (1), acontrol valve (9) in the main air duct (8) immediately upstream of thepreheating burner (10), a heating air duct (17) which branches off themain air duct (8) and leads into the flue gas duct (15) located betweenthe dust precipitation device (13+14) and the gas turbine (5), a controlvalve (19) in the heating air duct (17), a heat exchanger (20), providedbetween the control valve (19) and the flue gas duct (15), for heatingthe air flowing through the heating air duct (17), and a heating airdischarge line (21) which connects the flue gas duct (12) immediatelyadjoining the fluidised-bed boiler (1) to the off-gas duct (16). 2.Pressure-fired fluidised-bed boiler installation according to claim 1,characterised by heating coils (23) for additional heating of parts (12,13, 14, 15, 16) of the installation, and a governor (18) which emitssignals, depending on the speed of rotation of the gas turbine set(4+5+6), to the final control element of the control valve (19).